In wireless communication networks in which a moving portable radio may be communicating with a fixed base station, the received signal may be severely degraded by the many paths the signal may take from the portable to the base station. The many paths may include reflections from hills, buildings and overhead wires. Because the signal takes many paths, different reflections will arrive at an antenna slightly separated in time, and the combination of the multiple signals at the antenna may result in the cancellation of the signal for a period of time. Such a loss of signal is called a fade. The length of the fade will depend on the frequency of the signal and the speed at which the portable is moving. The problem caused by this signal degradation is called the multi-path problem.
The solution of the multi-path problem has received a great deal of attention by researchers. In part this is because of the increasing use of cellular telephones, whose use suffers greatly from the multi-path problem. Signal degradation caused by multiple paths of the signal increases the probability of error in the reception of the signal by altering the phase of the received signal. This is important because the emerging standard for digital cellular communications in North America uses a phase modulation scheme, namely .pi./4 offset differential quadrature phase shift keying (DQPSK). See Cellular System, "Dual Mode mobile station-base compatibility standard." EIA/TIA, Project no. 2215, Electronic Industries Association, January 1990.
With mobile radio channels short term fading may result in fades or nulls about every one half wavelength of the wave in question (.apprxeq.0.175 m for 850 MHz waves). If this pattern is assumed to be stationary, for simplicity, then an antenna mounted on a vehicle travelling at 27.7 m/s(.apprxeq.100 km/hr) will encounter a null about every 6.3 ms. With FM modulation, this leads to click noise at about 159 Hz. With DQPSK having 24.3 kSymbols/s, this noise may appear not to be long enough to introduce a symbol error. However, if the differential phase of the standing wave pattern is composed of long intervals of constant value (corresponding to intervals of constant amplitude) and short intervals of spikes (corresponding to nulls in the amplitude) say of 1% of the 6.3 ms, then the antenna will traverse a spike in 0.063 ms every 153.1 symbols, long enough to introduce a symbol error.
The multi-path problem has been approached in several ways. Several approaches have considered envelope modelling with little emphasis on phase modelling. Also, neural networks have been proposed as in Provence, U.S. Pat. No. 4,885,757 issued Dec. 5, 1989. Several researchers have done phase modelling of an rf channel, but have not looked at phase prediction using the amplitude. Also, phase estimators using open loops have been proposed, but these have focused on the Gaussian channel. Estimation and tracking of phase changes of a carrier signal has been carried out by the use of phase locked loops (PLLs). Phase locked loops may include a squaring amplifier, a feedback loop and a divide by two amplifier. In a phase locked loop a signal of the same frequency as the carrier is generated locally using a voltage controlled oscillator. The phase of the output of the VCO is made to track the phase of the received carrier through a feedback loop. In this manner, the phase of the incoming signal may be tracked.
However, phase locked loops are not of great use for example in the proposed North American digital cellular standard because they cannot track the incoming signal fast enough and because with information on the signal, the operation of the phase locked loop is disturbed. To operate a phase locked loop with information on the signal, the signal must be averaged or have the information filtered out first, thus adding to the complexity of the receiver.
Other techniques for phase tracking include sending a pilot tone which allows the receiver to synchronize its local oscillator to the carrier frequency. However, the use of pilot tones also adds to the complexity of the receiver because an additional signal must be sent, received and processed. It would be preferable to be able to extract phase information from the amplitude of the received signal itself.
However, it has previously been considered that a fading channel is not a minimum phase channel and that there was no relationship between the envelope and phase of radio signals transmitted over such channels.
However, the inventors have discovered that it is possible to relate the amplitude and characteristics of the phase of a radio signal transmitted over a fading channel, and that an estimation of the phase differential may be made from sampling the amplitude of the transmitted signal. The estimated phase differential may be used to modify or demodulate the received signal. The inventors have published their findings in "Estimation of Phase Differential of Signals Transmitted Over Fading Channels", Electronics Letters, Vol. 27, No. 20, 1823-1824, September, 1991.
The invention may be applied to a signal in which the information in the signal is carried in the time domain or in the frequency domain. In the case of the frequency domain, a frequency transform may be taken of the signal and the method applied to the amplitude values for the frequency transform. In either case, it is preferred that, firstly, a differential of a function of the amplitude determined to produce a data record, and then a transform of the differential is taken to produce a signal corresponding to the estimated phase differential of the data record. This produces an estimated phase differential with sign ambiguity which may then be resolved using known techniques and the received signal may then be modified using the estimated phase differential to produce a corrected signal.
Preferably, the differential may be computed by using the logarithm of the amplitude samples, and uses adjacent amplitude samples. The preferred transform is the Hilbert transform, and the phase estimation is computed in the case of the time domain from EQU .DELTA..phi.(t)=H[.DELTA.ln(A(t))]
in which H denotes the Hilbert transform, .DELTA..phi.(t) is the estimated phase differential, and A(t) is the amplitude samples. In the case of the frequency domain, "t" in the above expression and other expressions used in this patent document is replaced by "f".
Alternatively, the phase estimation may be obtained by:
constructing data frames of a number of consecutive amplitude samples of the electromagnetic signal; PA1 selecting segments of the data frames where the amplitude of the electromagnetic signal is at least a predetermined number of dB less than its mean; PA1 for each segment, estimating the phase differential .DELTA..phi.(t) from EQU .DELTA..phi.(t).apprxeq.-t.sub.0 /(t.sub.0.sup.2 +t'.sup.2)
where t'=t-t.sub.min, t is the time from the beginning of the segment, t.sub.min is the time in the segment when the absolute value of the signal reaches its minimum, and t.sub.0 is the period of time from the instant the amplitude of the electromagnetic signal reaches its minimum during the segment until the amplitude reaches double its minimum during the segment.
For simplicity, the sign of the phase differential need only be determined for segments with phase differentials greater than a preselected threshold.